This invention is in the field of polythiols. More particularly, it is in the field of urethane polythiols which can be made by reacting: (a) a mono-, di-, or trihydric polythiol (including a dithiol) with a polymeric isocyanate which is a mixture of methylene diphenyl diisocyanate (MDI) and higher analogs (or homologs) thereof; or (b) a di- or trihydric polythiol (including a dithiol) with a diisocyanate of the type recited in Embodiment A, infra.
Certain urethane polythiols are taught by U.S. Pat. application Ser. No. 408,338 (which is assigned to W. R. Grace & Co.), filed Oct. 23, 1973, and now abandoned, and by U.S. Pat. No. 3,883,598 (Guthrie et al, 260/609R).
MDI and the higher homologs thereof including admixture of MDI and such homologs are well known to those skilled in the art. Such admixtures (which are known as "polymeric isocyanates" and which are commercially available) are described by W. C. Bedoit, Jr. in an article entitled "Polymeric Isocyanates, What are They?" which appeared on pages 1-4 of "Urethanews", Vol. 1, No. 2, (Jan. 1972), The Martin Sweets Company, Inc., Louisville, Ky. 40201.
Bedoit's article teaches that the term "polymeric isocyanate" is a condensation of the term "polymethylene polyphenylisocanate" and that commercially available polymeric isocyanates are actually mixtures of a number of different polymethylene polyphenylisocyanates. Said article further teaches that methylene diphenyl diisocyanate (MDI), which has a functionality of 2, is the simplest molecule present in a mixture comprising a polymeric isocyanate and that such mixture also contains a trimer having a functionality of 3, a tetramer having a functionality of 4, a pentamer having a functionality of 5, plus higher molecular weight molecules (analogs) having higher functionalities.
MDI generally constitutes about 50% of such polymeric isocyanate.
In general polymeric isocyanates have an average functionality of about 2-6; however, this can be varied by increasing or decreasing the ratio of MDI to the higher (trimer, tetramer, pentamer, etc.) polymers in a mixture comprising a polymeric isocyanate.